Please post a screenshot of the error. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Is this file an image in your signature?

No, It is just a graphic that I put into the default email template. Don't remember where the default template is anymore since I did this a while ago. Here is the screen shot. The mail merge doesn't work anymore either since the update. I have to add each address one at a time. Kinda makes mail merge useless. Thanks for the help.

Check the TB52 release notes. https://www.mozilla.org/thunderbird/52.0/releasenotes/

They mention a setting where you can change the behavior.

Moscoso said

No, It is just a graphic that I put into the default email template. Don't remember where the default template is anymore since I did this a while ago. Here is the screen shot. The mail merge doesn't work anymore either since the update. I have to add each address one at a time. Kinda makes mail merge useless. Thanks for the help.

The template appears to be a blank message with your contact details, and this is usually accomplished with a signature that is automatically attached to every message you send, although it's possible to include it with replies and forwards, or on a message-by-message basis.

As mentioned by christ1, the method of inserting images has changed, and has resulted in some annoying features even for images stored on the local computer. At the moment, it's advised to rebuild templates and signatures in TB 52 with locally-hosted files to possibly avoid prompts to unblock the images.

As for Mail Merge, I notice that it is not yet specified to work with TB 52, so if you want to revert to TB 45 in the meantime, it is available here.